Vehicle pollution control

An example of Gaussian dispersion case A Chimney emits SO2 at a rate of 40 g/s. The stack diameter and height are 5 m and 150 m respectively. The stack gas velocity is 10 m/s and stack gas temperature is 140 OC. Ambient temperature is 25 OC. The atmospheric condition is cloudy with wind speed at 2.5 m/s at 10 m and 4.3 m/s at stack height. Calculate the ground level concentration of SO2 at 4 km downwind at plume centerline.

Internal combustion engine Combustion chemistry C8H18 + 12.5 O2 + 47 N2  8CO2 + 47 N2+ 9 H2O For complete combustion: Air-fuel (A/F) ratio = 15:1 CO is produced from incomplete combustion HCs are produced when the flame front approaches relatively cool walls of the cylinder. NOx is produced from N2 + O2 2 NO

Internal combustion engine combustion cycle

Petrol and diesel engine: Air-fuel ratio: higher ratio for diesel Compression ratio: High for diesel CO HC NOx PM Petrol High Low Diesel

CO and HCs control Modify engine design to improve combustion Use of catalytic converters

Control of NOx emission Generally more difficult to control, mainly to decrease combustion temperature Retardation of spark Decreased compression ratio exhaust has recirculation

Control technologies New control technologies Alternative fuels dual and three-way catalytic converter (NOx ->N2) Alternative fuels Conventional gasoline contains a mixture of paraffinic, and aromatic HC compounds Octane rating: When combustion is too rapid, a sharp metallic noise called knock is produced. Component that reduces the knock has the octane quality. Historically lead alkyls have been added to boost octane ratings. In unlead gasoline, aromatic HCs are added.

Control technologies Reformulated gasolines and oxygenated additives oxygenated additives such as MTBE, ETBE, methanol, ethanol (to boost the octane rating due to reducing aromatics) - Alcohol fuels Alcohol-gasoline blend - Compressed and liquefied gases Natural gas (mainly contains methane) Liquefied petroleum gas (largely propane) limited supply and higher exhaust reactivity

Control of diesel vehicle pollution in Hong Kong Euro III standard for vehicles of 1995 and onwards Existing ones Taxies: LPG Light buses: to be switched to LPG or electricity Heavy duty: traps and/or catalytic converters

Stationary sources control Tall stacks Fuel-use changes Fugitive emission containment Pollution prevention Substitution Process equipment changes Plant operating practices Maintenance Process changes Energy conservation “End of the pipe” control (see next pages)

Particle Collection Systems Cyclone collectors In general, collection efficiency increases with increased particle size and density, dust loading, and collector size. Multiple tubes collectors can be used to increase collection efficiency Cyclones are often used to control relatively large particles, and often are used in series as pre-cleaners for more efficient collectors.

Filtration Fabric filters are commonly used to control particle emissions where dust loading are higher, particle sizes are small, and high collection efficiencies are needed. Limitations include high capital costs, flammability hazards for some dusts, high space requirements, flue gas temperature limited to 285 C, and sensitivity to gas moisture.

Electrostatic precipitator High collection efficiencies for all particle sizes, low operating and power requirements; But high capital costs and space requirement.

Wet scrubbers Open spray Tower Venturi Scrubber Almost exclusively used to remove fine particles Capital costs are low, but operating costs are high (energy and disposal of waste liquid)

Control of gas-phase emissions More complex technologically than control of particles. Control techniques for a specific gas pollutant needs to be developed. Combustion Adsorption Absorption Direct flame incinerators

Control of gas-phase emissions Thermal incineration Additional fuel needed Catalytic incineration

Control of gas-phase emissions Adsorption Gas pollutant being adsorbed by the surfaces of solids such as molecular sieve, activated carbons etc Absorption Absorbed by liquids